This invention relates to high pressure discharge devices and particularly to an improved envelope for devices such as mercury arc lamps which have particular application in the manufacturing of television picture tubes and in integrated circuit fabrication.
Commercially available mercury arc lamps comprise a sealed fused quartz envelope forming a chamber and having lead-in conductors sealed in and extending a short distance through the ends of the envelope and terminating in electrodes of the same material as the lead-in conductors. The lamps further comprise pools of mercury contained within the envelope and positioned in the ends of the envelope surrounding the electrodes. Such lamps exhibit a limited useful life of about 100 hours because of devitrification of the fused quartz envelope. Devitrification means a change in the glass-like properties of the envelope material, particularly the transparency but it also includes and is not limited to changes in brittleness and hardness. Devitrification begins near the mercury pools at the ends of the envelope and spreads toward the center of the envelope during lamp operation. After about 100 hours of operation, devitrification has progressed to the extent that the luminous output of the lamp is reduced below a useful level and the lamp must be replaced.
Early workers in the lamp field believed that devitrification was caused by excessive arc temperature (see U.S. Pat. No. 2,321,178--Bourne et al.). Others believed that devitrification was caused by impurities forming centers of recrystallization which could be controlled by reducing the particle size of the impurities and homogeneously dispersing these impurities throughout the quartz material used to form the lamp envelope prior to resolidification (see U.S. Pat. No. 2,904,713 and U.S. Pat. No. 3,128,169--Heraeus et al.). Still others recognized that the presence of hydrogen shortened lamp life and constructed double walled bulbs including an outer bulb of hard glass encasing a quartz lamp therein. An inert gas atmosphere was included between the quartz lamp and the hard glass outer bulb to prevent diffusion of hydrogen from the ambient atmosphere through the hot wall of the quartz envelope (see U.S. Pat. No. 2,611,833--Noel). In another embodiment a glaze was applied to the exterior surface of the quartz in an attempt to reduce hydrogen diffusion and prevent devitrification (see U.S. Pat. No. 2,568,459). A publication by J. F. Waymouth entitled Electric Discharge Lamps, (p. 184, Cambridge, Mass., The MIT Press, 1971) and a report by C. Kenty, entitled The Liberation of Hydrogen and Carbon Monoxide In Glass by Ultraviolet Radiation, (p. 138, report delivered at the 1953 MIT Physical Electronics Conference and published in the conference proceedings) discussed the dissociation of OH radicals in glass and the diffusion of hydrogen through quartz. Despite the amount of information available concerning devitrification, this body of information is insufficient to permit the construction of mercury arc lamps that do not devitrify after a short period of operation. Devitrification continues to be a major cause of limited lamp life.
The terms fused quartz and fused silica as used in this disclosure mean if naturally occuring quartz crystal is melted to make glass, the material is called fused quartz but if a synthetic or a condensed silicon dioxide is melted, the material is synthetic fused silica.
The present inventor has discovered that devitrification cannot be eliminated by simply using fused quartz of high purity or synthetic fused silica of still higher purity. To eliminate devitrification it is necessary to establish and control both the total metallic impurity content and the OH radical content of the envelope material.